Backstage after Tuesday night's (Oct. 30) results show, in which host
Tom Bergeron broke the news that Derek's
neck injury had forced him to the sidelines, his partner
Shawn Johnson told us, "Losing Derek today was heart-wrenching ... Relating as a competitor and an athlete, I knew how hard the decision was for him."

"I've never seen that look on his face," she added. "It was just pain and frustration. It crushed me."

Having
Mark Ballas, her winning Season 8 partner, "step in is great," she notes. "I'm glad he did, I'm comfortable with him, but under the circumstances it's hard." (Mark partnered this season with
Bristol Palin, before they were eliminated Oct. 16.)

And although Derek's out this week, he's still Shawn's official partner. "He's going to be in all the rehearsals helping choreograph and making sure it's under his control if he steps back in Monday," she confirmed.

"It's still Derek in the rehearsals -- it's Derek's choreography, it's Derek's creative," explained the Olympic gold medalist. "Mark will just stand in as the rehearsal, the movement of it."

"You don't let another person step in to take over everything unless you're completely bowing out," she said. "And Derek is a fighter, he's as stubborn as I am. I know if he's not dancing on Monday, he'll be dancing next week for sure."

"I hope," she added. "I can only hope."

The three-time mirror-ball trophy winner's desire to win is "still there," insists his partner. "I think that's more so why he stepped out, preserving himself and making sure he's OK for hopefully later weeks to come."

Although Shawn's own injury -- her sprained knee -- was Monday night's major storyline, she told us, "That is the least of my worries right now! I'm wrapping it up and it's good."